Over the past few years, interest has grown in fast imaging techniques. In particular, the recent exploitation of contrast mechanisms based on physiology dependent microscopic susceptibility (T.sub.2.sup.*) effects has resulted in an explosive growth of the field of MR neuro-imaging, and has led to increasing demands on T.sub.2.sup.* sensitized fast MRI. Among the most popular techniques are echo-planar imaging (EPI), fast low angle--shot imaging (FLASH), and echo-shifted FLASH (ES-FLASH), each with its own advantages and disadvantages.
BURST imaging is a class of ultra fast imaging techniques initially proposed by Henning et al ("Fast Imaging Using BURST Excitation Pulses" in "Proceedings, 7th Annual Meeting, Society of Magnetic Resonance in Medicine, 1988", p. 238), and successfully implemented in a two-dimensional (2D) version by Lowe et al ("DANTE Ultrafast Imaging Sequence (DUFIS)", J. Magn. Reson. B 101, pp. 106-109 (1993)) on a small-bore animal scanner. A version with improved signal to noise ratio (SNR) has been proposed by Le Roux et al ("BURST Ultrafast Excitation Pulses" in "Proceedings, 7th Annual Meeting, Society of Magnetic Resonance in Medicine, 1991", p. 269).
Conventional BURST excites a set of equally spaced, narrow strips in an object, and creates an image from a single slice, perpendicular to the direction of the strips. In order to average, to scan multiple slices, or for three-dimensional (3D) imaging, repeated excitation of the same strips is required.
For ultra fast scanning, repetition times are short compared to the longitudinal relaxation time, leading to saturation effects and thus efficiency loss. In addition, when scanning in a 2D mode, the commonly used slice selective RF refocusing pulse also leads to additional saturation.
The present invention is directed to a method, (hereafter referred to as "Frequency-Shifted BURST" (or "FS-BURST")), which overcomes the above problems associated with prior art methods and allows for 3D volumes to be scanned within a few seconds.